Current fabrication techniques in the electronics industry require extremely precise, repeatable dispensing of very small amounts of adhesive or solder paste. For example, in surface mount technology drops of solder paste earlier applied to leads of 0.06 inch width and spacing of 0.100 mil now must be deposited to mount leads of 0.007 mil on spacings of 0.020 inch.
With such small, closely spaced leads a number of problems are aggravated. The amount of solder paste or adhesive deposited must be small and consistent in volume from lead to lead or the leads might be bent or encounter different attachment resistance. In addition, bridging can occur between the leads. Such attachment shortcomings are even more problematical when the surface mount chips which must be delicately removed, replaced and re-soldered have a value of up to $2000-3000. There are a number of pumps presently available for dispensing fluids but all have serious shortcomings.
Conventional syringe pumps, typically air pressure driven, operate (pulse) at high speeds, e.g., three times a second, which vibrates the paste and causes the solids to settle out and clog the pump. Further, the separation distorts the proper proportions of each of the elements--lead, tin, flux and solvent in the solder paste or constituents in other mixtures, so that an improper mixture of paste or adhesive is being deposited. Further, as the pumping action depletes the solder paste supply in the syringe, the empty space is filled by more and more air. Air is compressible; therefore, once the depletion begins there is no way to know just how much time and pressure should be applied on each stroke in order to keep the output volume consistent. Syringe pumps also are very difficult to control for small volumes. A typical syringe pump cylinder would have to move only micro-inches to dispense 10.sup.-6 cubic inches of material.
Systolic pumps squeeze the feed material. If the feed material is solder, then the flakes of tin and lead are pressed against the tube and their abrasive quality tends quickly to wear out the tube. Gear pumps suffer from the fact that the gear meshing action tends to squeeze the solder flakes or other feed material between the gear teeth. In the case of solder flakes, the lead or tin begins to coat the gear teeth. This eventually packs the teeth and jams the gears while removing lead and tin from the solder paste so that the paste dispensed does not contain its constituents in the proper proportions, and eventually jams the gears together.
In order to avoid this problem, gear teeth are typically designed with sufficient clearances so that the feed mixtures don't get crushed between the moving gear teeth. With these larger clearances the volumetric uncertainties are large with respect to the small volumes that these pumps are required to measure and dispense. In addition, the gear teeth characteristic dimensions must be larger than the clearances required. Thus with a reasonable tooth size, the amount of feed mixture dispensed requires only a small rotation of the gears, which is not easily controlled to give precise metering.
In conventional piston pumps, the feed pressure necessary to introduce the mixture to the pump can add unpredictable amounts to the volume being dispensed and the piston retraction can draw back the feed material, such as solder paste, from the nozzle, which further detracts from the predictability of the dispensing volume. It is necessary to vary the feed pressure in piston pumps to accommodate for differences in the viscosity of the feed material, but this introduces additional problems: increased pressure of the piston pump results in increased pressure tending to drive out the feed material, even when the piston is in the retracted position, so that leakage and inconsistency become even more of a problem.